1. Field of the Invention
The present invention relates to a combined oil control ring which is mounted on a piston of an internal combustion engine.
2. Description of the Related Art
A combined oil control ring having an approximately M-shaped cross section, which is configured such that upper and lower rails which bring outer peripheral surfaces thereof into slide contact with an inner surface of a cylinder and a web which connects the upper and lower rails and has a plurality of windows are integrally formed, is described in Japanese Unexamined Patent Publication Sho 61(1986)-45172 and Japanese Unexamined Patent Publication Hei 9(1997)-144881, for example.
As shown in FIG. 4, the combined oil control ring consists of an oil ring body 1 and a coil expander 2. The oil ring body 1 is an oil ring having an approximately M-shaped cross section in the radial direction which is formed of upper and lower rails 3, 4 and a web 5 which connects these rails 3, 4. Such a combined oil control ring is mounted in an oil ring groove 21 formed in a piston 20.
Outer-peripheral protruding portions 6, 7 of the respective upper and lower rails 3, 4 have an approximately trapezoidal shape which is formed of outerperipheral slide surfaces 8, 9 which are brought into contact with a cylinder wall 22 and upper and lower inclined surfaces 10, 11 which are contiguous with the outer peripheral slide surfaces 8, 9. Side surfaces 12, 13 of the respective upper and lower rails 3, 4 face respective upper and lower surfaces 23, 24 of the ring groove of the piston.
In the combined oil control ring, a lubricant which is scraped by the slide surfaces 8, 9 of the rails is allowed to move into an inner peripheral side of the oil ring body 1 from an outer peripheral groove 14 defined by the upper and lower rails 3, 4 and the web 5 through a large number of windows 25 formed in the web 5. The lubricant which is moved into an inner peripheral side of the oil ring groove 21 through the windows 25 is returned to an oil pan through an oil hole formed in the piston 20.
As shown in FIG. 4, inner peripheral surfaces 18, 19 of inner-peripheral protruding portions 15, 16 of the upper and lower rails 3, 4 have an arcuate shape and an inner peripheral groove 17 is formed of the inner-peripheral protruding portions 15, 16 and the web 5. The coil expander 2 is housed in the inner peripheral groove 17. By bringing the coil expander 2 and the arcuate surfaces 18, 19 of the inner peripheral groove 17 of the oil ring body 1 into contact with each other, the oil ring body 1 is pushed toward the cylinder wall 22.
The shape of the inner-peripheral protruding portions 15, 16 of the upper and lower rails 3, 4 may have a cross section of an approximately trapezoidal shape. In this case, as shown in FIG. 5, the inner peripheral groove 17 is formed of the inner-peripheral protruding portions 15, 16 and the web 5. The coil expander 2 is housed in the inner peripheral groove 17. By bringing the coil expander 2 and tapered surfaces 18, 19 of the inner peripheral groove 17 formed in the oil ring body 1 into contact with each other, the oil ring body 1 is pushed toward to the cylinder wall 22.
Here, as can be easily understood from FIG. 4 and FIG. 5, a maximum distance L2 in the axial direction between the inner-peripheral-groove arcuate surfaces 18, 19 or the inner-peripheral-groove tapered surfaces 18, 19 (see FIG. 5) of the oil ring body 1 is determined by a axial distance between inner peripheries 27′, 28′ of the inner peripheral groove 17.
As shown in FIG. 4, when the combined oil control ring, which the coil expander 2 is housed in the oil ring body 1, is mounted in the oil ring groove 21 of the piston 20, an opening between butt ends of the oil ring body 1 is widened until an inner diameter of the combined oil control ring becomes larger than a piston diameter. However, when the opening between the but ends of the oil ring body 1 is widened, it is likely to occur that the coil expander 2 falls off the inner periphery groove of the oil ring body 1. When the coil expander 2 falls off the inner periphery groove of the oil ring body 1, it is necessary to restart the assembling operation of the coil expander 2 and the oil ring body 1 from the beginning.
Recently, in addition to enhance the conformability to the cylinder wall, the wear resistance and the scuff resistance of the combined oil control ring, a demand for lowering tension of the coil expander is increasing. By lowering the tension of the coil expander, it is possible to obtain the excellent oil ring characteristics. On the other hand, the drawback that the coil expander falls off the oil ring body during assembling of the oil control ring to the piston becomes apparent. However, with respect to the combined oil control ring, no countermeasures have been taken to prevent the coil expander from falling off heretofore.